It is generally known that an antenna which is resonant with the frequency of the received signal is purely resistive and if it is properly connected by a feedline to a transmitter-receiver apparatus, no standing waves will occur on the feedline.
Peak performance of a multi-element parasitic antenna array depends upon proper phasing or tuning of the elements which can be optimally effective for a single frequency only. In the case of closely spaced arrays, which because of the low radiation resistance are usually quite sharply tuning, the frequency range over which optimum results can be obtained is in the order of only one or two percent of the assigned (resonant) frequency. In order to operate over a wide frequency range, the antenna is generally de-tuned thereby sacrificing some gain over the entire frequency spectrum.
The forward gain, along with the rejection of unwanted signals off the sides (front-to-side ratio) and the rejection of un-wanted signals off the back (front-to-back ratio) are highest when the antenna is operating at a resonant frequency. This holds equally for the antenna whether it is receiving or transmitting. If the frequency deviates from this resonant frequency point, the forward gain as well as the ability to reject un-wanted signals from the side or back rapidly deteriorates. When the antenna operates at its designated frequency point, un-wanted parasitic or harmonic signals are substantially attenuated.
Antenna structures with movable supports to change the physical as well as the electrical constants have been previously described. One such antenna structure is disclosed in U.S. Pat. No. 3,453,630 which operates in the UHF/VHF frequency range. In this known antenna structure, the physical size of the antenna is varied by means of pulleys together with a supporting structure. The supporting structure, however, is quite complex and the disclosure can be practical only in the range for which it was designed, i.e., in the high frequency range where the physical elements of the antenna structure are quite small, as compared with structure used in the low frequency ranges, for example, in the 10, 15 and 20 meter bands. For example, an antenna which is one wave length and adapted to transmit on 20 MHz would be 15 meters (47 feet) long, whereas an antenna operating at full wave length in the UHF frequency range, 300 MHz, would be 1 meter long (3.2 feet) long. Hence, the solution for the adjustment of an antenna operating in the UHF frequency range cannot be adopted readily for use in the lower frequency ranges because of insurmountable physical problems in constructing the antenna arrangements.